Years ago, the study of life beyond Earth was called “exobiology” or even “bioastronomy,” and the field was derisively referred to as “the science without a subject.” These days it’s “astrobiology,” carrying the imprimatur of NASA and many elite academic institutions. The discovery of distant planets, subsurface oceans on moons in the outer solar system, and exotic organisms known as “extremophiles” on Earth have boosted the field’s credibility.

Still, it’s a field that requires a great deal of inference and extrapolation. The one data point we have about life is what we see here on Earth. This is a good time to review some of the greatest milestones in the history of life on Earth. It’s an incredible tale.

The first and most obvious milestone was the origin of life. The details of that remain murky. Since life began, Earth has been transmogrified through cataclysmic events. The evidence of life’s origin was eroded, or vanished in the recycling of the surface through plate tectonics. The only thing left are grains of zircon and other minerals that may have signatures of molecules that hint of the presence of life.

“We don’t know anything about the origin of life,” George Cody, acting director of the Geophysical Laboratory of the Carnegie Institution of Washington, told The Post this week. He said the first organisms were probably obliterated by whatever came after: “The first living systems were totally overrun by their competitors, and their competitors were overrun by their competitors.”

As a result, he said, “We’ll never find a geological record of the origin of life.”

Confounding matters further, life may not have had a singular origin. It may have emerged multiple times in multiple places, and so incrementally that there may not have been a moment when non-life became life. Kate Adamala, who studies the origin of life and now teaches at the University of Minnesota, told us, “Most likely there was never a single point in history when something clicked and you went from no life to life. There was likely a very gradual and slow transition when there were more and more of those functions that we attribute to modern biological life.”

Chemistry happens. Complications ensue.

It’s the chicken-and-egg question writ large. If anyone asks you which came first, the chicken or the egg, tell them neither. There was always a precursor. Life is a process of modification and descent, rather than genesis. There was never a moment when an egg hatched a brand new thing called a chicken, or when a chicken produced, unexpectedly, something bizarre called an egg.

Once life got rolling, the Next Big Thing was photosynthesis, says J. William Schopf, a legendary paleobiologist at the University of California at Los Angeles. Photosynthesis, he told us, enabled organisms to create their own food from solar energy and at the same time transform the biosphere by emitting oxygen. For roughly half the Earth’s history, oxygen couldn’t linger in the atmosphere because it kept interacting with other gases and surface elements. But gradually the oxygen “sinks” filled up and oxygen, created by living things, became a major constituent of the air. That, in turn, enabled the emergence of life that could use oxygen metabolically, which supercharged the complexity of Earthlife.

The next big moment, Schopf said, was the appearance of sexual reproduction roughly a billion years ago. Until then, life merely cloned itself. Sexual reproduction is a technique for mixing genetic material rapidly in novel arrangements. That accelerated evolution, Schopf said. Soon enough we had multicellular organisms, followed by the Cambrian Explosion, 542 million years ago, in which all manner of hard-shelled creatures with elaborate body types evolved quickly.

After that, life was off to the races, doing all kinds of crazy stuff, including colonizing land. You had some mass-extinction events (end-Permian, end-Creteacous, etc.), but life always found a way to rebound.

“There are only a few things in the history of life that matter. The Origin of life. The development of photosynthesis and the change in the environment from that. The origin of sexual reproduction,” Schopf told us. “Life on this planet’s real simple. You can make it be being a plant-like thing or an animal-like thing. Our chemistry is all very simple.”

Then he amended his short list of things that matter: “I guess I would put intelligence in there as the fourth big thing that happened.”

The definition of intelligence is another slippery matter. Some folks will look at our current political news and question whether such an adaptation has yet arisen on this planet. But let’s stipulate that, on Earth, there is now a species capable of studying, through science, the origin and evolution of life. This raises the Anthropic Principle in the context of the Greenland fossil discovery, astrobiologist Sara Walker of Arizona State University told us.

If the structures found in Greenland are really fossilized microbial mats, then life was fairly complex early in the planet’s history and likely originated soon after the planet formed and cooled. That can be interpreted as a sign that life emerges quickly in general under the right conditions. But as Walker pointed out, we have to question some of our assumptions. We might expect that any planet with an intelligent observer looking for the origin of life will likely be one in which life began early – because on such planets life has had a very long time to evolve and diversify and give rise, eventually, to origin-of-life researchers. This philosophical conundrum is known as the Anthropic Principle. It’s kind of a cautionary note. It says: Extrapolate in moderation. Your very existence as an observer may lead to a misinterpretation of what is the norm.

With only one example of life – the stuff we see on Earth – we don’t really have a good, universally accepted definition of life. NASA some years ago defined life as “a self-sustaining chemical system capable of Darwinian evolution.” Not bad, but technically a single rabbit hopping around your garden is not alive, because by itself it can’t reproduce.

“I don’t think anyone has a definition of life that is universally agreed upon. The best we can do is describe some attributes of life – and then someone will poke holes in that,” says Pamela Conrad, an astrobiologist at NASA. “We know pretty well what non-life is, so we can rule out the other stuff. The leftovers is life.”